


Demon Slayer AU

by Fandom_Central_Director



Category: Haikyuu!!
Genre: Alternate Universe - Demon Hunters, Angst, Blood and Injury, Fluff and Angst, M/M, Other Additional Tags to Be Added
Language: English
Status: In-Progress
Published: 2020-12-19
Updated: 2020-12-23
Packaged: 2021-03-11 04:34:41
Rating: Mature
Warnings: Graphic Depictions Of Violence
Chapters: 2
Words: 3,925
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28179198
Author URL: https://archiveofourown.org/users/Fandom_Central_Director/pseuds/Fandom_Central_Director
Summary: Sakusa Kiyoomi's family has been murdered, his cousin is a demon. After learning what killed them, he has vowed to take them down by becoming a demon slayer.He gains allies and enemies, while keeping Komori safe from any slayers who feel they need to kill him(basically, Demon Slayer, but it's Haikyuu)
Relationships: Daishou Suguru/Komori Motoya, Komori Motoya & Sakusa Kiyoomi, Miya Atsumu/Sakusa Kiyoomi
Comments: 7
Kudos: 18





	1. Only Komori

Sakusa pulled the basket upon his back and looked back at his house.

“Okay, mom, I’m heading out!”

“Kiyoomi!” 

His younger brother and sister ran up to him, sparkles in their eyes. He rolled his eyes at the question they were about to plead to him.

“Can we go with you?!”

“I keep telling you guys, no,” He bent down, “It’s dangerous to go down the mountain in this snow, remember?”

They frowned, “Kiyoomi…!” They whined. He smirked and patted his little sister’s head.

“Stay calm. Don’t worry… next time, I’ll bring you guys along, okay?”

Their eyes lit up again, “Really?!”

He nodded. They jumped for joy and skipped back to the house. Sakusa chuckled and saw his cousin walk over to him.

“Hey, Motoya.”

“Hey, Kiyoomi,” Komori patted his shoulder, “Stay safe.”

“I will. I always come back, don’t I?”

“Yes… just stay safe.”

Kiyoomi nodded and turned to walk away. He slid down the mountain at some parts, and at the more level ground, he trudged through the snow. When he reached the town, he smiled at all the shopkeepers. He chatted with them, handing them pieces of coal he got a few days prior. When he was out of coal, he looked around for things he needed to buy for his family. 

When his time in the town was done, the Sun was setting, and he made his way back up to his house. It was dark when he was ¾ of the way there before he was stopped. 

“Hey, Kiyoomi!”

He turned to the small house that belonged to a family friend.

“Oh, hi, Mr. Washijou.”

“Come in, you’ll freeze and die if you stay out there.”

“I’m fine, it’s okay.”

“Kid, get the hell in here or I’ll make you come in.”

Sakusa scrunched his nose at the demand and looked back up the mountain. It was getting colder, and he heard from the people in town that it would be snowing even more after the sun went down. 

He sighed and walked into his house, giving him a half-smile. After eating what the man gave him, Sakusa slowly sipped his tea.

“You shouldn’t be out at night. Demons could get you.”

“And… you think demons are real?”

“Of course they’re real.”

Sakusa nodded and stared down at his cup. Of course, he didn’t believe him. Why would he? I mean, wouldn’t people be telling their children about this kind of stuff.

“Is there any way to kill the demons?”

“Yep. There are demon slayers protecting us from them, and kill them so they don’t kill us.”

Sakusa nodded and finished his tea. Afterward, he went to bed in a futon the man set up for him. He had a million and one questions… but he didn’t want to overwhelm him. He just closed his eyes, and slowly went to sleep.

When he woke up, he quickly packed up and said his goodbyes before leaving. He shuffled up the mountain, mumbling something to himself.

“Demons, huh? Why do people believe in this sort of thing? Of course, demons aren’t… I mean… they can’t, right?” Sakusa furrowed his brow, “Mom did say… doesn’t matter. All that matters is…” 

He gagged and covered his nose quickly. He smelled something horrible… and he smelled a metallic scent. Ever since he was little, he memorized that smell… it was blood. He dropped his basket and ran the rest of the way. The air was cold and clipped at his lungs, freezing them. It was hard to breathe like this. When he reached his house, his sister was lying on the ground, dead. Blood was seeping into the snow and felt like he could throw up. He observed the house further in and screamed at what was inside.

His family was slaughtered. His dad tried to protect his little brother, but both were dead. Cuts all over their bodies, their heads bashed in, blood splattered everywhere. He didn’t see someone there…

“Motoya… Motoya!”

He ran to his cousin’s room and he was lying on the ground, facing away from Sakusa.

“Motoya..?”

He didn’t answer. Sakusa felt the tears welling up in his eyes, and he turned away. To go back to the town, to the old man’s place, anywhere but where he currently was. He took a step when he heard it. A cough. He turned back and saw Komori was coughing slightly. He gasped and went to pick him up. Still warm, still alive.

Sakusa made quick work of carefully getting him on his back and quickly left the house. His home, murdered. Except for one. Komori. Only Komori.

He ran down the mountain, careful not to slip off the cliffs. 

“Don’t worry, Motoya! We’ll get there! We’ll get to town, and you’ll be okay! You’re gonna be-”

He tripped. Komori slipped from his hold, and even after trying to get a hold of him, Sakusa fell and was unable to grab him.

“Moto-”

He landed on the snow with a hard thud. He groaned and clutched his side. There was no blood. He sighed and sat up.

“Motoya? Motoya!” He stood up and saw his cousin standing at a tree. He was breathing heavily, refusing to look at Sakusa.

“Hey, Motoya,” He placed a hand on his shoulder, “Motoya, you don’t need to strain yourself. I’ll carry you, okay?”

Komori didn’t answer. Sakusa tried to move his head. There was a quiet growling emitting from him.

“Motoya? Are you-”

He was flung away by Komori. He rolled in the snow and looked up. Komori was suddenly over him, his teeth bared.

But they weren’t normal. His canines were sharper, and his face was twisted into one of hunger and anger. He leaned down, and Sakusa held up a stick, which Komori bit on instead.

“Motoya, what’s wrong?!”

Just a growl in response. Komori tried to overpower him, pushed himself down further, and biting into the stick more. Sakusa felt a weight growing on him, and when he looked down, he saw that Komori was getting bigger. He was going to crush him.

“Motoya! I know you’re in there! I know you’re scared and upset! Our family was just killed and now… and now you’re a demon who wants to kill his cousin… but that isn’t you! You can fight it! I know you can! You’ve always been able to go through hardship with a smile on your face! Fight it again! Please!”

He screwed his arms shut as he felt his arms getting weaker. He felt the tears welling up, but when he felt the water roll down his cheeks, it wasn’t his. He opened his eyes and saw Komroi was crying. He was no longer getting bigger, he actually began to shrink. He kneeled back, taking the stick with him. It rested in his mouth as he stared at the snow beneath him, crying. Sakusa hugged him, and his cousin were safe…

Something cut through the air, making Sakusa look up. High alert now. He looked around and saw someone was about to hurt Komori.

“No!” He pushed Komori away from the attack, and his hair was cut slightly. He rolled and glared at the person who tried to attack his cousin.

“Stay… the fuck away from us.”

The person didn’t respond. He just disappeared. Sakusa looked around but was unable to see him.

“Look down.”

Sakusa looked towards the voice, before glancing down. Where was Komori?

“Motoya!” He saw the person was holding him by his wrists. Komori was growling and trying to release himself from his hold. “Let go of him!”

“Why?”

“He’s my cousin, I need to take care of him!”

“This thing? He’ll kill you in an instant.”

“You don’t know Motoya! He’s a good person-”

“He’s a demon, not a person. And there are no good demons,” He held up his sword to his neck.

“Don’t hurt him!” Sakusa pulled an ax from behind him and charged at the man. He easily moved out of the way of Sakusa’s charge.

“You have some nerve charging at me.”

Sakusa glared at him, “ _ I  _ have some nerve? You’re the one who has the nerve, trying to kill my cousin!” He ran at him again, but this time, Suna attacked, kicking him in the stomach. Sakusa choked, collapsing to the ground.

“He’s a demon, he needs to be decapitated. Now.” He held his sword up again, but something landed right next to him. When the man looked, he saw that it was the ax he had with him. He didn’t even notice he threw it in the air.

“How did he-”

Komori ripped from his hold and ran towards Sakusa. Suna prepared to see the young boy be ripped apart but was surprised when he saw that Komori placed a hand over his leg protectively. He didn’t want to kill him… he was protecting him.

Komori snarled at him and was about to attack him before he was also kicked in the stomach. When the man knew they were both down, he dragged them to lie next to each other. He sighed and looked at Komori. He was calm before he intervened… his mouth… 

He took a piece of bamboo and a piece of cloth from his bag, and after chopping the bamboo into a smaller piece, he hooked the cloth through the piece and closed off the side creating a muzzle. He placed it in his mouth, and tied it around his head, securing it tightly.

“Mmn…”

Sakusa woke up and saw his cousin was alive and had a muzzle around his mouth.

“You awake?”

“Yeah… I am.”

“Good. Go to this house at the foot of Mt. Karasuno to see an old man. He’ll help you. Also, keep your… cousin out of the sun. It’ll kill him if he’s touched by sunlight, ‘kay?”

“...Okay.”

“Also, tell the old man that Suna Rintarou sent you.”

“Suna… Rintarou? Isn’t that Kansai? That’s almost opposite of Mt. Karasuno.”

“Just tell him.” He ran away from them in a blur, not giving Sakusa a moment to react. He looked at his cousin.

“I need to carry him… I need a new basket. But…”

He picked up his cousin, and they walked back to their house. He gathered each family member and buried them outside his house. He kneeled down and said his prayers to his family. He stood up, and took Komori’s hand, and led him away. He could see the mountain, and sighed at the long journey that would be ahead of them.


	2. Mount Karasuno

Sakusa nearly fell asleep while he was walking, but he panicked when he felt a bit of heat touch his skin. He opened his eyes and saw the sun was coming up. He looked at Komori who was walking beside him. He panicked, knowing that they couldn’t get him in the basket quick enough. He pulled Komori away, to a cave. Komori quickly ran inside, making sure he wasn’t touched by the sun.

“You okay, Motoya?”

He nodded, and Sakusa sighed. They would get nowhere with this method. He saw a sign which had a listing of a nearby town, and Sakusa cocked his head.

“Maybe… Motoya, I’ll be right back.”

Komori nodded and watched Sakusa leave. Sakusa followed where the sign pointed him and saw a small farm up ahead. He smiled and ran towards it. He saw some people working in the field, and he waved at them.

“Oh, hello there, young man.”

“Hi there. I was wondering…” He noticed a woven basket near their storage closet, “Could I have that basket?”

“Oh… sure, but it has a few holes-”

“That’s fine. Also, do you have any straw and a blanket I could have? I’ll pay you,” He held some money.

“That won’t be nessi-”

“I insist,” He pressed the money into the person’s hand. The items were brought to him, and he bowed before walking away. Before he got back to the cave, he wove the straw to give the basket extra strength and walked the rest of the way to the cave. 

“Hey, Motoya? I got this basket. Since I wanna get to walking during the day, you can stay here while I do. I also got this blanket so you don’t burn.”

Sakusa looked inside the cave and saw Komori had dug a hole while he was gone. His head popped up, worried look in his eyes.

“Here, try to see if you can fit.”

He brought the basket closer to him and watched his cousin attempt to crawl inside. But, Komori was too big to fit inside.

“Hm… remember when you grew bigger? Maybe you can get smaller,” He patted Komori’s back, “Can you hear me? Try to get smaller, see if you can fit then.”

Komori began to shrink, so he could fit inside the basket. He pushed it upright, and sat inside, looking up at Sakusa. His cousin smiled at him and tied the blanket around the basket. Sakusa pulled it upon his back and walked out of the cave, looking towards Mt. Karasuno. He breathed in the icy air, before exhaling, confident in how his trip would go.

He began the trek, walking back down the road to the farm where he got the basket. He waved at the people, and they waved back. He then devoted all of his focus and attention to the roads ahead of him. He was going to get to Mt. Karasuno, he was going to meet the old man Suna told him to meet, and he was going to get his cousin back the way he was. 

It was dark, and Sakusa decided to let Komori out of the basket. They walked up a mountain, looking at the trees and staring at the fresh snow. Sakusa noticed something out of the corner of his eye. It was a temple, and Sakusa’s eyes lit up.

“Hey, Komori, look! A temple! Maybe we can rest there.”

Komori nodded and followed his cousin to where he started to run towards. But, as he got closer, he heard something. Someone gnawing at something… flesh. Sharp teeth eating the flesh. His eyes went wide, and he pulled out his ax.

“Motoya, stay back!” 

He ran towards the temple and threw the door open, seeing three dead bodies. Covered in blood and blood caked the walls and floors. He gasped when he saw someone eating a fourth body. A demon. 

“Stay away from him!” He pulled out his ax and glared at the demon.

“Hah?! Who dares…” The demon glared back at Sakusa.

Sakusa didn’t blink before he saw the demon was right in front of him. In a panic, he sliced his throat, only scraping the surface. Thankfully, it took the demon off of him for a second, knocking him back. Sakusa quickly backed out of the temple, seeing if he could get away quick enough. 

He was unable to, getting hit by the demon. Sakusa fell back and landed in the snow with a hard thud and looked up at the demon.

“Damn...you don’t even have a sword, and yet, you got me,” he chuckled, “No matter. My cut’s already healing.”

Sakusa’s gaze was directed to the cut he created on the demon’s neck. It was healing itself, and when he was fully healed, he laughed at the horrified look on Sakusa’s face. 

“What the hell?”

“You didn’t know? Demons heal crazy fast. That way I can do this!”

The demon jumped forward and slashed at Sakusa. He jumped out of the way, but his cheek was cut. He hissed and looked up at the demon. He swung at him with his ax, but it was unable to protect himself. 

Komori ran over to where Sakusa was and saw the dead bodies. He stared at them, suddenly feeling a wave of hunger wash over him. He stood there, salivating, staring at the dead bodies that were strewn along the wall. He was almost unaware of what was going on behind him.

Sakusa was slashed again and was pushed into a tree. He landed with a groan and tried to reach for his ax. The demon stepped on his hand and snarled at him. Sakusa screamed, trying to retract his arm. He was unable to and he looked up, seeing an angry demon’s face.

“Shit.”

The demon kicked him in the face, knocking him into another tree. Sakusa coughed up blood and clutched his side.

“Agh- Moto-”

He was picked up by his hair and he was face-to-face with the demon. He huffed, steam exiting his nose, and Sakusa cringed at the action.

“He can’t help y-”

Komori tore himself from the bodies and kicked the demon’s head off his shoulders. He was growling at the demon’s body. He stopped when he was punched, however. He was kicked, punched, and scratched. He tried to fight back, but he was lethargic. His movements were getting slower and slower. He didn’t react fast enough when his collar was grabbed before he was thrown far away from Sakusa.

“Moto-”

“Don’t forget about me, bastard!” 

The head of the demon jumped at Sakusa, who defended himself with his ax. He was pushed back and glared at the head.

“Dammit! Motoya, I’ll be there soon!” He tried to get the head off of his ax, but it was impossible. He had a tight grip on the weapon and wasn’t letting go anytime soon. Sakusa knew there was no way he could actually beat the demon without a proper, useable weapon in his hands. He thought of the only thing, and he threw the ax to a tree, trapping the head between the ax and the tree.

He ran to his cousin, pushing back branches and bushes to see his cousin getting hit and kicked by the headless body. He was edging Komori near a cliff and filled Sakusa with anger and anxiety.

“Stay away from him!” Sakusa pushed the body off a cliff and watched him go over. However, he overshot how much he pushed him and felt him slipping off the side. He yelled, getting the attention of his cousin who grabbed his wrist, keeping him from going over the edge.

“Thank you, Motoya…”

He pulled himself up and over the cliff. He looked at the demon that had landed on the ground. He sighed and walked back to the demon’s head. He pulled out a dagger he saw on the ground and held it to the demon’s skull.

“That won’t kill him.”

He turned around and saw a man with a mask covering his face. He wore a black and orange montsuki and his hands were folded behind his back.

“Then… what do I do?”

“Figure it out.” 

Sakusa watched the man walk away, and stared at the dagger in his hand. If this wouldn’t work… his eyes trailed to a rock that laid on the ground. He sighed at the thought and picked up the rock. He mumbled something to himself, and raised it above his head, but hesitated. He didn’t want to smash down on his head… but what other choice did he have? If he didn’t… what would happen.

He looked out of the corner of his eye and saw that the sun was rising. He watched as the demon before he burned up and died, without a trace. He sighed and saw the old man burying the dead bodies. He walked over to him and cleared his throat.

“The demon’s… he- it’s dead…”

“You hesitated and he burned up in the sun.”

Sakusa tensed up and looked down at him, “Yeah… I did…”

“I don’t know why Suna sent you…”

Sakusa sighed and turned around. His wrist was grabbed and he saw the man. He turned back to him, and suddenly his head jerked to the side. He hadn’t realized he was slapped until he felt the sting rise up to his cheek. His hand rested over the mark, and he looked back up at the man.

“But I will train you.”

“Wait… really?”

“Yes,” He began to walk away from him, “Gather up your kid brother and follow me.”

_ ‘Cousin,’  _ Sakusa thought, but went to but Komori in the basket, cover him, and ran after the man.

“Wait!” The man stopped. “What’s your name?!”

He was silent for a few seconds, “Ukai Ikkei. Come on.”

He ran away quickly, and Sakusa ran after him.

“Ukai-san! Wait!” Sakusa tripped and gathered himself, before having to continue running. The old man was fast and silent. Sakusa couldn’t pick up a single footstep made by him, even when he strained his ears. It was weird…

They reached a house where Ukai stopped, and when Sakusa caught up to him, he was breathing heavily. 

“Ukai… san… what was… what was that… about?”

“This is where I will train you to be a proper demon slayer. Set your brother down.”

“He’s my cousin…”

“Then set your cousin down.”

Sakusa followed his instructions and set Komori on a futon, where he slept. He brushed his hair away and smiled softly at the boy on the ground.

“I’ll be back Motoya… don’t worry. Don’t go anywhere… I’ll always be back.”

Sakusa stood back up and followed Ukai who was outside. He walked up the mountain next to his house and made sure Sakusa was following him. They walked through the growing mist, stepping over branches, and with every step, Sakusa felt dizzier. He looked around and saw a few animals pass them.

He stopped when Ukai did and looked at him expectantly.

“I will train you if you can get down to my house before sunrise.”

And with that sentence, he disappeared. Salusa looked around for him, but he was gone. Sakusa sighed and paid attention to the leaves that were still crackling from when they walked up the mountain. All he had to do was follow the sound, and he’d be learning from this man. 

He dashed down the mountain, confident in himself. Until he tripped over a rope. He looked around and heard a snap, which made him duck in fear. He was lucky, because several knives flew at him, which would have cut him. He continued running, but this time, he fell into a hole. He groaned when he looked up and sighed to himself.

He had to be ready for any and every trap a demon might set, and he also needed to be on high alert. He stood up and climbed out of the hole. He also needed to get up quickly so he wasn’t killed. He continued down the mountain, getting hurt and hit, falling in traps, and quickly getting up. He was fueled by seeing his cousin and getting his cousin to get better. He knew he could. He knew it.

Ukai looked out the window and didn’t see anyone pass by as he watched the top of the sun rise over the horizon. He sighed and walked to his kitchen before his door slammed open. There stood Sakusa, bloodied and bruised, breathing heavy.

“You…”

“I am going to be a fucking demon slayer. Got it?”

He fell to his knees before falling asleep against the door frame. Ukai watched his soft form sleeping, tired, and dedicated. He was different. Very different. More different than… any student he had in the past. He was confident about him. 

“You will be. I can feel it.”

He picked him up and carried him to the empty room where Komori also slept.

He would be a demon slayer.


End file.
